Problem: In physics class, the girl to boy ratio is $2$ to $2$. If there are a total of $8$ students, how many girls are there?
A ratio of $2$ girls to $2$ boys means that a set of $4$ students will have $2$ girls and $2$ boys. A class of $8$ students has $2$ sets of $4$ students. Because we know that there are $2$ girls in each set of $4$ students, the class must have $2$ groups of $2$ girls each. There is a total of $4$ girls in physics class.